The Book of Lost Tales contains the first versions of the stories which would later become The Silmarillion, and is notable because, while very primitive in style and content, it is very close to the later work in many ways. Each of the Tales is followed by notes and a detailed commentary by Christopher Tolkien.

While many of the names in the book are identical or close to those in the later versions, some of them bear almost no resemblance to their final forms. J. R. R. Tolkien changed names rather frequently, sometimes with several new variants (rejected in turn) written in a single manuscript. Confusingly, sometimes the name applied to one thing is later used to refer to a different thing, the original use abandoned. As an example, the house of Elves called "Teleri" in The Book of Lost Tales is not the same as that in The Silmarillion (see Teleri). The original usage of "Teleri" would eventually change until the name became "Vanyar". Meanwhile, the house of Elves called "Solosimpi" would inherit the name "Teleri".

The framework for the book is that a mortal Man visits the Isle of Tol Eressëa where the Elves live. In the earlier versions of the "Lost Tales" this man is named Eriol, of some vague north European origin, but in later versions he becomes Ælfwine, an Englishman of the Middle-ages.

There are more changes visible within the book, and it is not internally consistent, partially because even while still writing it Tolkien began rewriting earlier parts as his ideas about the world changed. The Tales were eventually abandoned, but they were resurrected in part as the "Sketch of the Mythology" which would become the Silmarillion.

For publication the book was split into two volumes: The Book of Lost Tales 1 and The Book of Lost Tales 2, but this is simply an editorial division. Both volumes are separated into several "Lost Tales".

Contents of Book 1:

"The Cottage of Lost Play" — the "framework" story

"The Music of the Ainur" — the first version of what would become the Ainulindalë

The following is a direct comparison between the contents of the early Book of Lost Tales and the later sketches that became The Silmarillion.

Although one of the Lost Tales often corresponds to more tales from The Silmarillion, still the narrative in the Lost Tales is more extended (typically, 50% pages more).

In J. R. R. Tolkien's notebooks, the 4th and 5th tales were a single one told by Meril-i-Turinqi, but Christopher Tolkien decided to separate them because of their length. Same applies to the 6th and 7th tales, told by Lindo.